Towards routine shipborne measurements of columnar CO ₂ , CH ₄ , CO, and NO ₂ : a case study for tracking regional-scale emission patterns

Abstract. Mobile remote sensing observations from shipborne platforms offer a unique opportunity for validating satellite observations and sampling plumes of greenhouse gases and short-lived air pollutants from the world's highly populated coastal megacities and industrial sites. Here, we demonstrate the capabilities of a shipborne setup that combines a sun-viewing EM27/SUN Fourier transform spectrometer for the shortwave-infrared spectral range with a DOAS (Differential Optical Absorption Spectroscopy) spectrometer for the visible spectral range, enabling simultaneous measurements of the column abundances of carbon dioxide (CO2), methane (CH4), carbon monoxide (CO), and nitrogen dioxide (NO2). For several months in 2023 and 2024, the instruments were operating autonomously on a commercial vessel traveling back and forth along the coast of Japan. We show that, for CO2, CH4, and CO, precision and repeatability comply with the standards of the Collaborative Carbon Column Observing Network (COCCON). Further, for a case study in the vicinity of Nagoya, we demonstrate the scientific leverage of this mobile multi-species approach: Simultaneous measurement of CO2, CO, and NO2 enhancements is used to successfully disentangle emissions from different sources. Our study demonstrates that routine shipborne deployment is possible. The setup delivers highly precise and accurate concentration records of the target species, as required for satellite validation, and enables emission monitoring of sources due to their distinct emission ratios.